The present invention relates to a power factor correction circuit. More particularly, the present invention relates to a continuous current control type power factor correction circuit.
Reference is made to commonly assigned, U.S. patent application Ser. No. 08/083,233, filed Jun. 29, 1993. This prior application also relates to a continuous current control type power factor correction circuit. In general, the "power factor" of an electrical circuit is defined as the ratio of active power to apparent power. That is, when an AC voltage (E) and AC current (I) have a phase angle difference (.alpha.), the apparent power is EI and an active power is EIcos.alpha.. Therefore, the power factor (EIcos.alpha./EI) is expressed as cos.alpha..
One form of conventional power factor correction devices improve power factor by providing a load terminal, or an input terminal, to which power is supplied through such passive components as a resistor or a capacitor. Unfortunately, these conventional power factor correction devices have input currents which vary with their loading. Thus, even though power factor improvement is achieved for one particular system, the input current cannot be actively determined according to a change in the load or input voltage.
Another form of conventional power factor correction device obtains an improved power factor by controlling the current of a booster converter using the control circuit of a zero current switching (ZCS) system. For this type of power factor correction circuit, input current is controlled discontinuously along a full-wave rectified input line voltage. The input current is swung heavily from zero to peak value. Thus, noise, for example, hum, is generated and it is difficult to obtain a large average input current. That is, using a discontinuous current control type power correction circuit in a large power system is difficult.
FIG. 1 is a circuit diagram showing a conventional discontinuous current control type power factor correction circuit. In FIG. 1, the discontinuous current control type power factor correction circuit detects the state change of the primary winding current iL of a transformer 18 connected to a line 11 by employing the secondary winding current of transformer 18 and a resistor 24 and controls control switch 22. As shown in FIG. 2A, when the primary winding current of transformer 18 increases at a slope having the predetermined value shown in FIG. 2B by means of turning on control switch 22, the secondary winding voltage has the waveform as shown in FIG. 2C. Thereafter, the appropriate level voltage V.sub.mo, as shown in FIG. 3, where the divided value of an input line voltage V.sub.in and a voltage V.sub.L of load 34 are combined, and the voltage V.sub.cs corresponding to the primary winding current of transformer 18 detected by resistor 24, are compared by a comparator 44. When V.sub.cs is larger than V.sub.mo, control switch 22 is turned off. When control switch 22 is turned off, secondary winding voltage V.sub.2 of transformer 18 goes high, as shown in FIG. 2C, and the primary winding current of transformer 18 decreases at the slope shown in FIG. 2B. As the slope is decreased to zero, secondary winding voltage V.sub.2 of transformer 18 is held high, until reaching the zero point of the slope of the primary winding current, as shown in FIG. 2C. However, when the slope of the primary winding current iL of transformer 18 reaches the zero point, secondary winding voltage V.sub.2 immediately goes low.
When control switch 22 is turned on using the above-described voltage change, input current is increased, as shown in FIG. 3, by the quick switching operation of control switch 22. When control switch 22 is turned off, the input current is decreased. In other words, as shown in FIG. 3, the peak of controlled current follows the proportional value of the line voltage, and is shaped as a triangle wave. Accordingly, an average value of the triangle wave is a sinusoidal wave having the same phase as that of the voltage waveform.
Unfortunately, the discontinuous current control type power factor correction device adopting ZCS system is difficult to use for high power applications due to the following reasons. Input line current swings from zero to peak current as the input line current follows the waveform of the input line voltage, and thus, in order to have a large average value of an input current, the peak value of the input current must be greatly enlarged. As a result, the associated booster converter device is over-stressed. In addition, the resulting ripple produces a bothersome humming noise.